In a device for NMR measurements, a so-called probe head is located in a strong static magnetic field, usually produced by a superconducting magnet. The sample to be measured is inserted into this probe head. The probe head includes radio frequency (RF) coils or resonators which excite the nuclear spins and detect the signals generated by the excited nuclear spins. Since the nuclear spin signals are generally very weak, the signal-to-noise ratio (S/N) is always a very important, or even the most important performance criterium in nuclear magnetic resonance. This S/N ratio can be improved in a very efficient manner by cooling the receiving coil down to minimum temperatures thereby increasing the efficiency and at the same time minimizing thermal noise. Such probe heads are called cryoheads. The present invention concerns one aspect of cooling the receiving coil with the aim of keeping this temperature as low and stable as possible with simultaneous radio frequency irradiation onto another nucleus to minimize unnecessary deterioration of the S/N ratio and additional measurement artefacts.
Most cryogenic probe heads include a detection channel (e.g. 1H (protons)) and one or more so-called decoupling channels (e.g. 13C and 15N (isotopes of carbon or nitrogen)). They usually also have a locking channel for stabilizing the magnetic field, which uses deuterium (2H). A probe head of this type is called a triple-resonance-inverse head (TXI).
A probe head of this type is explicitly discussed in the embodiment below which is not to be regarded as a limitation, rather an example of the inventive teaching. All other combinations of detection and decoupling nuclei are possible. In particular, it is also possible to examine the same nucleus with two channels and therefore with two RF coils at the same frequency, one channel for transmitting and the other for receiving.
For a TXI head, it is important to maximize the sensitivity of the detection channel (1H). During normal operation, the decoupling nuclei (13C and 15N) are used only for decoupling and magnetization transfer. Such a probe head typically comprises two RF coils. An inner coil at which the detection channel is disposed, and a second outer surrounding coil at which the X channels (13C and 15N) are disposed. The locking nucleus may be circuited along with either the inner or the outer coil.
In conventional probe heads, the sensitivity (i.e. the signal) is optimized by disposing the detecting coil as closely as possible to the sample, i.e. at the very inside around the sample. The cryoheads are additionally optimized by cooling the RF coils to low temperatures, typically <25K. Typically, the pre-amplifier is also cooled to prevent its noise contribution from becoming overproportionally large compared to the very low thermal noise of the receiving coil. It may be operated at temperatures of e.g. 77K or less.
In a cryohead, heat is typically removed from the coil system via a cold gaseous He cycle. A preferred embodiment uses a Gifford-McMahon Cryocooler to cool the cold He cycle. Such coolers usually have two stages to obtain the required low temperatures.
The pre-amplifier-in,-accordance with prior art is also advantageously cooled to prevent unnecessary increase in noise. The pre-amplifier is also conventionally and advantageously installed in the cryohead to minimize S/N losses through the signal lines between the RF coil and the pre-amplifier. The pre-amplifier is also conventionally cooled by the cryocooler.
U.S. Pat. No. 5,889,456 (FIG. 3a of U.S. Pat. No. 5,889,456) describes a particularly favorable embodiment which provides a large degree of flexibility and is schematically shown herein in FIG. 2. The cooling power of a two-stage Gifford-McMahon Cooler (coldheads) 6 is thereby tapped at both stages 7 and 8 and the cryohead 1 has two cold sinks (heat exchangers) 38 and 39 at different temperatures. The cold is typically transported from the cooling system 5 to the cryohead 1 via gaseous helium which circulates through the two cooling loops 9 and 10. The warmer temperature [approx. 77K] is conventionally used to cool the pre-amplifiers and structural elements in the cryohead using the warmer cycle 9, which helps to reduce the required cooling power for the cold cycle. This is, in particular, possible since the cooling power at the warmer stage 7 of the coldhead 6 is substantially larger than at the colder stage 8. The cold cycle 10 [<25K] is then used to cool the RF coil arrangement with minimum loss in thermal efficiency and temperature.
The sample 2 to be measured is located in a so-called central tube 22 which is permeable for the radio frequency fields and which extends through the cryohead. This tube and the sample 2 are approximately at room temperature. The detection coil 11 and the decoupling coil 12 are cooled to working temperature (<25K) via the heat exchanger 13. A vacuum 3 is established inside the cryohead which ensures the required thermal insulation of the cold parts from the central tube and therefore of the sample and to the outer surfaces of the cryohead.
The pre-amplifier 4 is also cooled. The coil heat exchanger 13 is cooled by the cooling loop 10 (“cold cooling loop”) and the pre-amplifier is typically cooled to 77K using the cooling loop 9 (“warm cooling loop”).
The cold is generated in the cooling system 5 in the coldhead 6 and is connected to the cryohead through the likewise evacuated transfer line 40. The coldhead 6 typically comprises two stages, the first stage 7 having a temperature of approximately 40–77K and the second stage 8 having a temperature of <25K. The cooling medium, He gas in the present case, is driven by a compressor (not shown) which is operated at room temperature and is connected to the cooling system via connections 41. The He gas is initially at room temperature and is pre-cooled by the counterflow heat exchanger 32 and then further cooled by the staged heat exchanger 38 of the first coldhead stage 7. It then passes through the second counterflow heat exchanger 33 where it is further pre-cooled, and to the staged heat exchanger 39 of the second coldhead stage 8.
The minimum temperature is thereby reached. The He then passes into the cryohead where the heat exchanger 13 is cooled as described above. After the He has returned to the cooling system, it passes via the counterflow heat exchanger 33 and then through the cooling loop 9 to the head where it cools the electronics. Upon returning to the cooling system 5, it cools the incoming gas at the counterflow heat exchanger 32 and leaves the cooler at connection 41, returned to room temperature, for further transport in the compressor.
The throttle valve 34 forms a bypass for optional regulation of the cooling power of the cooling loop 9.
U.S. Pat. No. 5,889,456 provides an exact thermal analysis of such a system. A system of this type provides excellent function, saves cooling power and is also very robust and undemanding in terms of required thermal design.
It is also possible to cool the RF coils and the pre-amplifier(s) with only one cycle. The cooling details of the pre-amplifier electronics have no primary relevance to the present invention and are not explained in more detail herein.
The disadvantages of prior art systems are as follows:
There is an inherent contradiction between the functions of the two coils: First of all, the detection coil 11 should always operate at minimum temperatures to keep the thermal noise at a minimum. However, the decoupling coil 12 generates large RF fields and is therefore exposed to high RF powers. These are largely dissipated in the coil itself which is therefore exposed to high thermal power. In case of decoupling, powers on the order of magnitude of 10W are dissipated over long time periods. These powers form the main thermal load of the cryohead while the detection coil (except for the comparatively very low radiation losses towards the sample) requires nearly no or only comparably little cooling power. The decoupling powers PD(t) are typically applied in pulses (FIG. 3). (The individual power pulses 50 may consist either of a continuous power (CW) or have a further internal structure, i.e. consist of a series of successive pulses (e.g. CPD=Composite Pulse Decoupling). This is, however, not important in the present case. It is only important that a high power is dissipated during the time interval 50). A simple example, which is by no means exhaustive, will be used to illustrate these embodiments. In accordance therewith, decoupling takes place at a duration of 200 msec during the time intervals 50, i.e. an RF power is applied to the decoupling coil and a pause 51 of 0.8 sec passes until the spin system has relaxed again. This,produces a repetition period 52 of 1 sec and a duty cycle D of 0.2 or 20%. Many other pulse sequences are also used. The duty cycle may, in particular, be considerably higher.
The detection coil should operate at as constant a temperature as possible during the entire experiment to prevent noise changes or increases during operation. It is also possible that the inherent resonance of the coil depends to a certain degree on the temperature which could cause frequency changes in the coil during pulsed operation of decoupling which could produce disturbing phase modulation artefacts in the resulting spectra.
The detection coil 11 is conventionally thermally cooled by the same heat exchanger 13 as the decoupling coil 12. To prevent variations of the coil temperature, the temperature of the heat exchanger 13 is conventionally kept constant through active regulation, which consists of a temperature sensor 36 and a heater 37, via a regulation system (not shown) (FIG. 4).
This function is satisfactory at first glance but entails a series of disadvantages upon exact analysis, which result from the following facts:
1. inefficiency of the heat exchanger 13
2. limited thermal transport capacity of He gas
3. the temperature control is only static (average value) and can generally perform no dynamic control.
4. dynamic control would make sense only to a limited degree, since it would require regulation to the maximum, very high temperature.
5. it is very difficult to measure the actual temperature of the RF coil itself and its control would consequently also be difficult, in particular in the presence of large temporal variations.
The above-mentioned points will be discussed below.
Although the control may indicate a constant temperature, the actual temperature dependence of the RF coils is not that simple, as is shown in FIG. 5 for a conventional arrangement.
The time dependence of the decoupling power PD(t) is shown in the upper representation of FIG. 5. The lower part shows the temperature increase of both RF coils ΔTc (compared to a case without decoupling).
In any event, the average power {PD} results in an increase in the average temperature which is indicated by TAVE.
This produces a certain average temperature TAVE. If the control point of the regulator is set at this or a higher temperature, the probe head can be kept constant at this temperature with or without decoupling.
This, however, is not the entire situation. If the temperature dependence is observed in a time-resolved dynamic manner, one obtains the result shown with the broken lines TDyn.
It must thereby be noted that due to the very fast heating, the control system cannot readjust during this time. If this were possible, only the variation of the temperature would be eliminated. Towards this end, the set point would have to be moved to an extremely high asymptotic temperature TASy which would not lead to an improvement in the absolute (maximum) temperature (see below).
During the pulse, the coil components and the heat exchanger 13 are rapidly heated, since a very high power is applied during this time which greatly exceeds the average (CW) power (factor 1/D).
Use of the pulse power on the decoupling channel therefore initially produces a considerable temperature increase (FIG. 5 bottom) whose steepness is given by the specific heat of the heat exchanger and the coil arrangement. For a high specific heat C of the coil component and the heat exchanger 13, the heating is moderate before the cooling phase starts again, as is indicated by TDyn(1).
If C is small, the temperature rises quickly as is indicated by TDyn(2). The temperature reaches the maximum asymptotic temperature TASy already within the decoupling period and more or less stabilizes. This value is much higher than the average temperature TAve (due to the factor 1/D).
It is thereby disadvantageous that precisely during this time 50, the acquisition (data acquisition) takes place at the receiving channel, i.e. exactly when the noise of the detection coil and therefore also its temperature should be minimum.
The specific heat of the conventional heat exchanger materials (metals) considerably decreases with decreasing temperatures in the temperature range of interest. For systems which operate at low temperatures, this specific heat C thereby decreases more and more thereby approaching the extremely high TASy within the decoupling pulse.
The considerably higher temperature during acquisition can greatly exceed the average and instantaneous temperature. Moreover, the temperature of the receiving coil changes greatly during operation which can produce tuning errors and, phase changes during acquisition and may lead to serious spectral phase errors which cannot be corrected.
The conventional thermal behavior of the detection coil is therefore actually much worse than the steady average control temperature of the regulation system, and is therefore certainly less than ideal. The actual temperature of the detection coil can, in particular, be substantially higher than the theoretical temperature thereby reducing the actual performance and spectral quality.
It is the object of the present invention to eliminate the above-mentioned disadvantages of prior art, in particular, to ensure a minimum and constant absolute temperature at the receiving coil.